1. Field of the Invention
The present invention relates to a transmitter, a receiver, a communication system, and a communication method which employ a spread spectrum communication technique.
2. Description of the Related Art
In recent years, communication systems which employ the spread spectrum communication technique have attracted considerable attention. The spread spectrum communication technique is a technique for communication whereby the baseband signal is spread over a wide bandwidth by means of a spread code. It is characterized by enhanced privacy and excellent capabilities in eliminating interference. Since the same data can be transmitted simultaneously over the same bandwidth, it is employed in diversity communication systems. An example of a conventional communication system of this type is disclosed in Japanese Laid-Open patent application 8-65201.
In this example of the prior art, the receiver is provided with two reception systems, and reception quality is improved by selecting the output of the system having the better reception quality.
While this conventional communication system does afford improved communication quality, it requires dual reception systems and dual transmission systems, and therefore entails a more complex design and higher equipment costs. The performance required of a system, the environment in which it is used, and similar factors generally differ from one system to another, and it is not the case that extremely high communication quality is required in all systems. Furthermore, where a plurality of terminals communicate with a single base station, the performance requirements differ from terminal to terminal. For example, terminals located in environments characterized by high levels of fading and noise require implementation of diversity in addition to the spread spectrum communication technique in order to ensure adequate communication quality, while other terminals not subjected to the effects of these to the same degree do not require diversity, and the spread spectrum communication technique alone may suffice. In the latter case, one of the two transmission systems and reception systems is idle. This cannot be said to represent an efficient communication system.